Vehicle manufacturers build and sell vehicles with entertainment systems to meet customer demands. It is known that a vehicle manufacturer may use the same model of core entertainment device on multiple vehicle models. Such core entertainment devices typically employ identical core functional blocks such as signal amplifiers, tone controls, and input selectors. The vehicle manufacturer may also variously add accessories such as CD players, DVD players, I-pod docks, and memory stick readers as signal sources in accordance with the desires of a vehicle customer purchasing the vehicle. In addition, entertainment systems typically have auxiliary input jacks so the vehicle customer can readily connect other signal sources. Each vehicle model may employ a different ground reference point for grounding the core entertainment device, and each signal source may have a distinct ground reference point that is different from the ground reference point used by the core entertainment device. The device connected to the auxiliary input jack may be connected to a ground reference in the vehicle through a 12V power adapter or cigarette lighter adapter, or may be powered by internal batteries and so has no connection to a ground reference. If the device connected to the auxiliary jack is connected to the 12V power adapter, the ground connection may be a direct low resistance connection, or may be a higher resistance isolated connection. Furthermore, the device connected to the auxiliary jack may require a voltage other than 12V so may include a linear type or switching type power converter. As such, the connection of a signal source to the core entertainment device may have an unknown grounding scheme and may be susceptible to ground noise corrupting the signal.